Rotary jetting tools are commonly used to clean scale or other deposits from oil and gas production tubing. These tools may also be used to drill soil and rock formations. In submerged applications such as deep well service, the effective jet range is severely limited by turbulent dissipation. The jets must be located at a large angle from the axis of rotation to minimize the standoff distance between the jet and the formation. Multiple jets are required to ensure that all of the formation ahead of the tool is swept by the reduced range of the submerged jets. An over-center jet must be placed so that its axis is directed across the rotary axis of the tool. Jet quality is also important, especially in harder formations. Large upstream settling chambers and tapered inlet nozzles improve jet quality by reducing inlet turbulence. It is desirable to make the rotary jetting tool as short and compact as possible to enable the tools to pass though tight radius bends in tubing, or to pass through a short radius lateral exit window from a well. In these applications, the tool may be mounted on a flexible hose. Finally, there is a need to provide a speed governor on the tools to prevent runaway. Unfortunately, the design requirement for compactness is in conflict with the other above-identified design requirements.
Rotating jetting tools may use an external motor to provide rotation, or the rotor can be self-rotating. A self-rotating system greatly simplifies the tool operation and reduces the tool size. In a typical self-rotating system, the jets are discharged with a tangential component of motion, which provides the torque necessary to turn the rotor. Most self-rotating systems use a sliding seal and support bearing to allow rotation of the working head. A drawback associated with this configuration is that the torque produced by the working jets must be sufficiently great to overcome static bearing and seal friction. The dynamic friction of bearings and seals is typically lower than the static friction, so the rotors can spin at excessive speeds, which can cause overheating or bearing failure. Most self-rotating jetting systems also incorporate a thrust bearing. Such bearings are subject to high loads and failure when the rotary speed is too great.
Hydrodynamic journal bearings rely upon a thin film of fluid that supports the rotating shaft through hydrodynamic forces. Journal bearings cannot support high thrust or radial loads, but are effective at high velocity—where the hydrodynamic lift is greatest. The thrust load can be eliminated with a balanced, or floating, rotor design. The rotor shaft is supported by opposed radial clearance seals, which also act as hydrodynamic journal bearings. If the shaft diameter is the same on both ends of the rotor, there is no thrust due to internal pressure of the fluid. This approach has been used by Schmidt (U.S. Pat. No. 4,440,242) and Ellis (U.S. Pat. No. 5,685,487) to provide a self-rotating jet. In both patents, the working fluid is introduced from the tangential surface of the rotor shaft to the center of the rotor by crossing ports. The drawback to this configuration is that the fluid settling chamber is small compared with the sealing diameter of the rotor. Also, the jet forming nozzles must be drilled from outside the rotor and do not produce a good quality jet. Finally, the jets discharge at a relatively small exit radius and small angle from the tool axis so the standoff to the gauge of the tool is relatively large. In the Schmidt patent, a separate rotor head that extends well beyond the thrust-balanced section is provided. The rotor head can be made relatively large to accommodate the desired jet pattern, but this approach defeats the requirement for a compact tool.
The rotational speed of a radial bearing rotor may be too high for effective jet erosion drilling of rock. A speed governing mechanism would substantially improve the jetting performance. Mechanisms incorporating mechanical, viscous, and magnetic brakes have been used to govern jet rotor speed. These mechanisms are typically relatively long and complex. It would therefore be desirable to incorporate a simple, compact speed governor in the rotor.
An important application for jet drilling rotors involves drilling short radius holes. The jet rotor required for such an application must be as short as possible to enable the tool to negotiate tight comers and short radius bends. Thus, it would be desirable to provide a compact jet rotor with multiple jets in orientations that: (1) generate sufficient torque to reliably start the rotor; (2) ensure efficient drilling; and, (3) eliminate side forces on the radial bearing that can cause wear. It would further be desirable to provide a compact jet rotor incorporating relatively large internal flow passages within the jet rotor, to minimize upstream turbulence and pressure losses, in order to provide the best possible jet performance. It would be still further desirable to provide a compact jet rotor incorporating an integral and compact speed governing brake. Finally, it would be desirable to provide a compact jet rotor incorporating wear-resistant materials in the design with sufficient precision to enable reliable manufacture and performance.